The coating frown is a common width-wise uniformity defect caused by too little flow being distributed to the ends of a hopper away from the inlet. The coating frown is a distribution, in the case of a center-fed hopper, where there is more laydown of the coating composition in the center than at the two ends, resulting in the frown shape. For an end-fed hopper, the frown is one-sided, with less laydown away from the inlet.
The frown profile can result from any geometry of cavity and slot. The two extremes of geometry are the straight, untapered inner cavity 10 as shown in FIGS. 1 and 2, and the coat hanger inner cavity shown in FIG. 3. The straight cavity 10 is a general purpose cavity for coating a variety of fluids and flow rates, whereas the tapered coat hanger cavity 20 is less flexible, often optimized for a particular flow condition and particular theology of the coating composition. The coat hanger cavity 20 also has less fluid stagnation than the straight cavity. In FIGS. 1-3, the inner cavity is in communication with the outer cavity 11 through communication slot 13. The coating composition is discharged through metering slot 8.
The frown profile occurs because of the transverse pressure drop in the cavity. It occurs in the tapered cavity hopper when the flow condition deviates from design flow condition. Additionally, it occurs when the rheology of the coating solution deviates from the design rheology. FIGS. 1-3 show an end-fed hopper where the coating composition is introduced at one end of the hopper 12. However, the coating frown discussed earlier occurs in a center-fed hopper, as there is a transverse pressure drop in the cavity from the center to each end of the cavity.
The present invention eliminates or minimizes the frown profile by providing an adjustment or control feature on the hopper.